How to join 2 or more sculpts into one?
Use the Boolean Modifier. Or if you want to have a better workflow use the BoolTool addon. Tutorials on how to use them are enough out there and its quite simple but I will explain it nevertheless.


Add the Boolean Modifier to the first object you want to merge. 
Click that little eyedropper in the object box of that modifier.
hover over the object to merge with and press LMB
Apply the modifier and delete the mesh that you selected with the eyedropper


All of that can be done with the BoolTool addon with one click on the Direct Union button